1. Technical Field
The present disclosure generally relates to uninterruptible power supplies. More particularly, the present disclosure relates to compact uninterruptible power supplies that use a multi-level two stage dc-dc converter and a multi-level inverter to supply power from an energy storage device.
2. Background of Related Art
There is large demand for data centers to store data due to the emergence of Web-2.0-enabled businesses in the financial, e-commerce, pharmaceutical, and multi-media industries. The digital storage market doubles every 18 months, which translates to an annual growth rate of approximately 150% for the next 5 years. Computer equipment manufacturers continue to expand their data collection and storage capabilities of their servers, which are widely used in data centers across the world. This expansion has led to an increase in the total power requirements of data centers both while connected to an electrical utility and during an interruption in power from the electrical utility. In particular, data centers now demand power in the megawatt range and voltage in the kilovolt range. As a result, data centers require uninterruptible power supplies (UPSs) that can meet these high power and high voltage requirements when there is an interruption in the power supplied from the electrical utility.
Over the past ten years, the cost of copper has increased approximately 400% (from about $0.77/lb to about $4/lb). By using medium voltage (6.6 kV or 13.8 kV) distribution, it is possible to reduce the size of the copper power supply cables, thereby reducing the cost of the power supply cables. It is also possible to reduce the critical power losses between the utility grid and the server computer rack by under 5% by using a transformerless medium voltage (MV) UPS and using a MV distribution system.
In boost mode, the DC-DC converter for the energy storage device of a UPS may use a single power semiconductor device to step up the voltage provided by the energy storage device, e.g., a battery, in the UPS. However, a single power semiconductor device is not available to step up the output voltage of the UPS so that it can connect across medium-voltage lines, for example, 6.6 kV or 13.8 kV AC lines. Therefore, the AC output of UPSs typically uses a step up transformer to step up a voltage of a battery. For example, the transformer may step up the voltage of a battery at 700 V DC or some other low voltage to the AC voltage of the power supplied by the utility supply, for example, 13.8 kV or some other medium voltage.
FIG. 1 shows a system 100 for supplying power to information technology (IT) and/or mechanical load 155 according to the prior art. The system 100 includes a utility/generator power supply system 195 and a UPS 115 that includes a step-up transformer 140. Under normal load conditions, power is supplied to the load 155 entirely by the utility supply 165. The utility supply 165 supplies an AC voltage ranging from about 3.3 kV to about 13.8 kV. The mechanical portion of the load 155 includes electrical power required to operate cooling equipment required to remove waste heat generated by the IT portion of the load 155.
A surge protector 180 is used to limit voltage spikes in the power supplied by the utility supply 165. A bypass line 162 allows maintenance tasks or other work to be performed on system 171-173 when ON/OFF switch of bypass line 162 (not shown) is closed and a static transfer switch (STS) 175 is opened. Line filters 170 are coupled to each AC line 171, 172, and 173 to reduce harmonics in the power supplied by the generator 160 or the utility supply 165. The STS 175 supplies power to a step-down transformer 150 when the STS 175 is closed. The step-down transformer 150 can convert the medium voltage supplied by the utility supply 165, e.g., 13.8 kV, to a low voltage, e.g., 400 V. The low voltage is then supplied to the load 155 having an appropriate current level.
When an interruption or disturbance in the power supplied by the utility supply 165 is detected, the STS 175 opens and the UPS system 115 starts supplying about 100% of the power to the load 155 via the UPS's transformer 140. The UPS system 115 can supply power to the load 155 for a short period, e.g., approximately five minutes, but generally the generator 160 starts generating power if the interruption is more than a few seconds.
The UPS system 115 generates power from a low-voltage energy storage device 105, e.g., one or more low density lead-acid batteries B. The low voltage VB of the energy storage device 105 can range from about 300 V to about 600 V. The low voltage is then converted to a high voltage, e.g., approximately 700 V, by a bidirectional DC-DC converter 110. The bidirectional DC-DC converter 110 includes one stage for converting the low voltage DC to a high voltage DC. The high voltage DC is then converted to a low AC voltage, e.g., approximately 400 V, using a two-level inverter 120.
The AC voltage output from the two-level inverter 120 passes through filter 130, such as an inductor-capacitor (LC) filter, to a step-up transformer 140. The step-up transformer 140 converts the low AC voltage to a medium AC voltage, e.g., about 13.8 kV. The medium AC voltage output from the step-up transformer 140 is then provided to the step-down transformer 150, which converts the medium AC voltage to a low AC voltage, e.g., about 400 V, that is appropriate for the load 155.
Once the generator 160 has reached its reference speed and stabilized, transfer switch 190 shifts the primary power source from the utility supply 165 to the generator 160. During this shift, the output voltage of the UPS system 115 is synchronized to be in phase with the output voltage of the generator 160. Once the STS 175 is closed, a soft transfer from the UPS system 115 to the generator 160 is executed until the load 155 is entirely powered by the generator 160. The energy storage device 105 of the UPS system 115 is then recharged by the power generated by the generator 160.
After the power interruption or disturbance ends, the load 155 is shifted from the generator 160 to the UPS system 115 because the utility supply 165 may be out of phase with the generator 160 and the STS 175 shifts the primary power source to the utility supply 165. The output voltage of the UPS system 115 is then synchronized to be in phase with the output voltage of the utility supply 165. Once the output voltage of the UPS system 115 and utility supply 165 are synchronized, the load 155 is quickly transferred from the UPS system 115 to the utility supply 165. Then, the energy storage devices 105, e.g., batteries B, of the UPS system 115 are recharged from the utility supply 165 so that the UPS system 115 is ready for future interruptions or disturbances in the utility supply 165.
The step-up transformer 140 in the UPS system 115 meets the power requirements of the load 155; however, the step-up transformer 140 is a large and bulky component of the UPS system 115. As a result, the power density of the UPS system 115 is lower because the transformer 140 occupies a large amount of floor space, which, in some cities, can be quite expensive. The transformer 140 also introduces considerable losses (approximately 1 to 1.5% of the power) into the system thereby reducing the efficiency of the UPS system 115. Also, when traditional sinusoidal pulse width modulation (PWM) technique is used to operate the inverters and an ON-OFF PWM technique for bi-directional single stage DC-DC converters 110, current distortion increases. As a result, LC filters 130, which are expensive and bulky, are placed at the output of the two-level inverters 120 to reduce the current distortion or harmonics as demanded by the IT and/or mechanical load 155.